Throughout this specification, the phrase "top hat" cross-section will be used to denote a laser beam cross-section having a sharply defined edge at a target plane. A graph of the intensity of a beam having a top-hat cross-section, versus position along a line in the target plane, will have a sharp-edged appearance as shown in FIG. 1(a). In contrast, the graph of the intensity of a beam having a Gaussian cross-section, versus position along a line in a target plane, will have a bell-shaped ("Gaussian") appearance as shown in FIG. 1(b).
Throughout this specification, the phrase "defocused" beam profile will be used to denote a beam profile in which the beam waist (i.e., the location along the beam profile at which the beam has its smallest cross-section) does not coincide with the target plane.
Ophthalmic laser beam delivery systems are designed to deliver a laser beam to a treatment site in a patient's eye (for example, to a desired location on the retina at which photocoagulation is to be performed). Typically, visible radiation also illuminates the treatment site to enable the physician to view a magnified image of the site. In a class of ophthalmic laser beam delivery systems in widespread use, both the treatment beam and a separate illuminating beam are directed along a common path through a microscope assembly (known as a "slit lamp") to the patient's eye.
Throughout the present specification, in the context of ophthalmic surgery, the phrase "target plane" will denote a plane through a treatment site in a patient's eye.
Two types of conventional ophthalmic beam delivery systems, each of which delivers a laser beam from an optical fiber through a slit lamp to a patient's retina, are known as "parfocal" and "defocus" systems. A parfocal system includes optics for focusing a sharp image of the fiber core (i.e., the fiber near field) to a spot (having size selectable from a range of spot sizes) on the patient's retina. The beam projected by a parfocal system through the eye (including the cornea) and onto the retina also includes fiber far field radiation, which has a Gaussian cross-section. The beam produced by a parfocal system has the disadvantage in that, when a large, sharped-edged fiber core image is focused on the retina, an undesirably high beam power density is often present at the cornea. This is because the beam divergence is low when the fiber core image size is large, giving a beam of nearly the same diameter through its entire transit of the eye (as illustrated in FIG. 11, to be discussed below).
The problem of small beam diameter and therefore high power density at the cornea typically exists when the sharp-edged fiber core image focused on the retina has large size. In some cases, depending on the type of fundus lens used during laser treatment, the corneal power density can exceed the power density at the retina with large, parfocal spot sizes.
An ophthalmic beam delivery system of the "defocus" type typically produces a beam whose waist is positioned behind the patient's retina. The image of the fiber core is also focused in a plane behind the patient's retina. Such a beam will have relatively low power density at the cornea, but undesirably has a Gaussian cross-section at the retina (due to the optical fiber far field radiation output characteristics).
The beam delivery system described in U.S. patent application Ser. No. 07/604,585, filed Oct. 26, 1990 (and assigned to the assignee of the present application) can be adjusted to operate either as a parfocal or a defocus system. However, it does not eliminate the above-described disadvantages of conventional parfocal and defocus systems.
Until the present invention, it was not known how to eliminate the described disadvantages and limitations of conventional ophthalmic beam delivery apparatus, of either the parfocal or defocus type.